Many MEMS applications require tilting motion of reflecting surfaces (i.e., micro-mirrors). In particular, there are applications with the need for tilting motions in two directions simultaneously, i.e., a mechanism having two degrees-of-freedom (DOF). One of such application is a scanning micro-mirror device for the use of displaying images.
Micro-mirrors offer numerous advantages in realizing optical scanning systems. Their small size, low cost and low power consumption provide a compelling solution for image creation and display systems. However, current state-of-the-art design still fall short on achieving the required performance (i.e., resolution, scan range, repeatability, scan linearity and power) which is required to making micro-mirrors based displays competitive to other display technologies.
The actuation of micro-mirrors in two DOF is essential for the functionality of the device. One way to implement actuation of an element in two DOF is with two different elements, each of which moves simultaneously in orthogonal directions. One way to implement actuation of an element in two DOF is by actuating a single gimbaled element having two DOF. The preferred architecture for micro-mirror scanners is the gimbaled design, where a single mirror is manipulated across two DOF. This architecture utilizes only one mirror for the two dimensional scan, thus reducing the chip size and simplifying the optical system design. The mirror is manipulated across both axes by using an actuation mechanism. The scan across one axis (horizontal axis) is done at a relative high frequency, typically a few KHz, while the scan across the second axis (vertical axis) is done at a relative lower frequency, typically a few tens of Hz.
Actuation Mechanisms
The prevalent actuation mechanisms are:                a) Electrostatic, where capacitance change induces an electrostatic force to move the mirror about an axis. Typically, comb drive actuators are used to create this movement.        b) Electromagnetic, where alternating current in a magnetic field induces a magnetic force to move the mirror. Most commonly, the mirror has current carrying coils, and is positioned inside a magnetic flux created by fixed magnets which are placed beside the mirror and coil unit.        c) Piezoelectric, where a piezoelectric material is used to translate voltage into mechanical force and consequently, mirror movement.Electrostatic Mechanisms        
Reference is made to FIG. 2 (Prior art), which illustrates a typical prior art electrostatic actuation mechanism 100. A mirror 110 is affixed to moving element 120 (rotor) having an axis 122. Two electrodes 130 (stator) are place below each end of element 120 and when a different electrical potential is introduced between element 120 and an electrode 130a, a force F is created, attracting element 120 to electrode 130a, thereby creating a movement of element 120 about axis 122. When movement is required in the opposite direction, an electrical potential different is introduced between element 120 and the other electrode 130b. The electrostatic actuation mechanism 100 also creates some force f on axis 122, which typically, in MEMS technology, is flexible, and thus creating an unwarranted movement of axis 122 in the direction of force f. The unwarranted movement of axis 122 is a result of the electrostatic actuator 100 being non-symmetric. Furthermore, the usage of electrostatic actuators 100 in two DOF introduces more problems. Typically, micro-mirrors 110 are designed to operate at their resonant frequency (i.e. the frequency at which the mechanical structure oscillates). However, the scan linearity and repeatability in display applications is greatly affected, which causes pixel and thereby image blurring and distortion. Moreover, in most of the prior art work, a single actuator is used to excite motion in both scanning axes. As a result, there is a mechanical coupling of the two DOF (i.e., actuation of one DOF also induces some residual actuation force on the other DOF), which directly affects the scan linearity and the image sharpness and reduces the elements operation quality and efficiency. Various solutions have been proposed to this problem; however none provides a suitable solution to the problem of attaining a linear scan at low power consumption. U.S. patent application 2004223195, by Nomura, is an example of a gimbaled mechanism with two DOF using electrostatic actuators.
Electromagnetic Mechanisms
Reference is made to FIG. 3 (Prior art), which illustrates a typical prior art electromagnetic actuation mechanism 200, including a magnet 210 and an element 220 having an axis 222 is wound with a coil 224. When a DC electric current is introduced into coil 224, a repelling/attracting force 226 is induced relatively to the magnetic field of static magnets 210 and the DC electric current, thereby creating a movement of element 220 about axis 222 in the direction of the repelling/attracting force 226. When movement is required in the opposite direction, the polarity of the alternating electric current is introduced into coil 224 is changed, thereby inducing force in the opposite direction.
The main advantage of the electromagnetic actuation is the high force density, resulting in a device that can operate in protective environment without the need for vacuum. However, it is not trivial to use electromagnetic actuation for inner gimbaled moving elements. Therefore, it is prevalent to use electrostatic actuation for the above. Although a method that can simultaneously actuate a gimbaled element in two DOF, while using two different actuators, is more robust and less sensitive to mechanical coupling, but is not trivial for implementation.
Symmetric Electrostatic Mechanisms
To overcome the asymmetry of electrostatic actuation mechanism 100, a different electrostatic actuation mechanism was introduced in U.S. Pat. No. 6,595,055 (U.S. '055), given to Schenk et al. U.S. '055 provided a scissors-like mechanism that introduced an electrostatic actuation mechanism with a pure torque applied to the axis of movement of the rotor, not giving raise to unwarranted force on the axis of rotation.
Reference is made to FIG. 4 (Prior art), which illustrates a symmetric prior art electrostatic actuation, with scissors-like mechanism 150. A mirror 160 is affixed to moving element 170 (rotor) having an axis 172. Electrostatic actuation mechanism 150 also includes a stator element 180, whereas there is some angle θ0 between stator 180 and rotor 170, when there is no electrical potential different between stator 180 and rotor 170, i.e. V1(t)=V2(t). When a difference in electrical potential is introduced between stator 180 and rotor 170, a force F is created, attracting rotor 170 to stator 180, thereby creating a movement of rotor 170 about axis 172. In this embodiment no residual forces are applied to axis 172. However the mechanism introduced by U.S. '055 has manufacturing difficulty as both stator 180 and rotor 170 are created from the same layer of silicon, which raises the problem of applying V1(t)≠V2(t) in the same layer of Silicon. U.S. '055 provides a solution, which is difficult to manufacture, where the stator layer includes two additional sub-layers: an insulating sub-layer and a metal layer to which V1(t) is applied.
Feedback Control
A critical parameter in micro-mirror design is the attainable scan angle, which determines the optical system design and resulting size of the display. One of the main limitations in all actuation mechanisms is the maximum attainable scan angle since current or voltage, at the micro-mirror are limited.
To provide repeatability and linearity, a feedback mechanism is incorporated in the mirror design. The feedback mechanism however is susceptible to interference from the drive signals which are typically orders of magnitude stronger. Furthermore, the feedback control of existing scanners falls short of the required linearity and repeatability and typically sense one DOF.
Conclusion
Thus, there is a need for and it would be advantageous for applications using micro-mirrors architecture to have a system that can meet one or more of the following challenges:
a) Eliminating the coupling/interference/crosstalk between the two axes of motion;
b) Achieving low drive power while maintaining a linear and repeatable scan;
c) Increasing available drive force to increase scan angle:
d) Improving the feedback sensors to increase the resolution; and/or
e) Optimizing feedback algorithms to provide the required repeatability and linearity.
The invention described henceforth, presents a new paradigm in actuation schemes and architecture of gimbaled elements, which eliminates the mechanical coupling of the two DOFs. This invention enables a simple implementation and sufficient power for high quality performances typically required in such devices.